Atrial fibrillation (“AF”) is a heart arrhythmia wherein the atria of the heart beat chaotically thereby providing generally poor conduction of blood into the ventricles of the heart and hence reducing the flow of blood throughout the body. AF has been shown to lead to long-term health problems such as increased risk of thrombolytic stroke. AF can also cause reduced cardiac efficiency, irregular ventricular rhythm and unpleasant symptoms such as palpitations and shortness of breath. In some cases, AF can trigger ventricular fibrillation (VF) wherein the ventricles of the heart beat chaotically thereby providing little or no blood flow to the brain and other organs. VF, if not terminated, is usually fatal.
Hence, it is highly desirable to terminate AF. The current, most common therapy for atrial fibrillation is the administration of anti-arrhythmic drugs that control atrial and ventricular rates during atrial fibrillation. However, these drugs can actually be proarrhythmic, causing the arrhythmia to worsen. At best, anti-arrhythmic drugs appear to provide short-term therapy. Another technique for terminating AF is to administer an electrical cardioversion shock to the atria of the heart. The cardioversion shock, if successful, terminates the chaotic pulsing of the atria and causes the atria to resume a normal beating pattern. Patients prone to AF may have an ICD implanted therein capable of detecting AF and automatically administering one or more cardioversion shocks to terminate AF. Typically, about two joules of energy is administered within each cardioversion shock at an initial voltage of between 100 to 500 volts (V). The duration of the pulse is usually between 5–15 milliseconds (ms). State or the art ICDs are also capable of detecting a wide variety of other heart arrhythmias, such as VF, and for administering appropriate therapy as well. For VF, the ICD administers a much stronger cardioversion shock (referred to as a defibrillation shock) directly to the ventricles of the heart. The defibrillation shock has at least ten to twelve joules of electrical energy. Note that, herein, “cardioversion” refers to the delivery of any electrical shock intended to synchronize action potentials of myocardial cells within the heart to terminate arrhythmias. Defibrillation refers to a type cardioversion specifically intended to terminate fibrillation.
Although atrial cardioversion shocks have been found to be effective for terminating AF within many patients, the shocks can be quite painful. One reason is that the patient is typically conscious and alert at the time the shock is administered. In contrast, the much stronger ventricular defibrillation shocks for terminating VF are typically not administered until the patient has lost consciousness and hence the patient may feel only residual chest pain upon being revived. Because AF is not usually immediately life-threatening, painful cardioversion shocks for its treatment are often perceived by patients as being worse than the condition itself and therefore not tolerated. Indeed, anxiety arising from the fear of receiving a painful cardioversion shock may be sufficient to raise the heart rate sufficiently to trigger the shock. As some patients have hundreds of AF episodes per year, techniques for reducing the pain associated with cardioversion shocks are highly desirable. It is also desirable to reduce pain associated with ventricular defibrillation shocks. Although patients receiving ventricular defibrillation shocks are usually unconscious when the shock is delivered, in some cases, such shocks are erroneously delivered while the patient is conscious due to a false-positive VF detection, resulting in considerable patient pain.
One method for reducing pain arising from cardioversion shocks involves altering the stimulation waveform of the shock to, for example, reduce or smooth initial voltage peaks. See, for example, U.S. Pat. No. 5,906,633, to Mouchawar et al., entitled “System for Delivering Rounded Low Pain Therapeutic Electrical Waveforms to the Heart.” Although waveform alternation techniques are promising, pain reduction typically requires a reduction in either the total shock energy or the peak shock voltage and, as such, may likewise reduce the effectiveness of the shock.
Another method for reducing pain arising from cardioversion shocks is to deliver a pre-pulse pain inhibition (PPI) pulse prior to the main shock. See, U.S. Pat. No. 6,091,989, to Swerdlow et al., entitled “Method and Apparatus for Reduction of Pain from Electric Shock Therapies.” With PPI techniques, a relatively weak stimulus (the PPI pulse) is applied to the patient shortly before the main cardioversion shock. The nervous system responds to the weak stimulus in a manner such that the pain associated with the subsequent main shock is reduced or otherwise inhibited. The PPI pulse is usually either electrical or acoustic. Insofar as electrical pre-stimulus is concerned, PPI techniques have heretofore typically employed either a single relatively long, low voltage PPI pulse or a single relatively short, high voltage PPI pulse. The long, low voltage PPI pulse is usually delivered at about 12–20 volts (V). The shorter, high voltage PPI pulse is usually delivered at the voltage of the subsequent main cardioversion shock. Each has its respective advantages and disadvantages.
A significant advantage of generating a short, high voltage PPI pulse at the same voltage as the main shock is that only a single shocking capacitor is required, precharged to the main shock voltage. To instead deliver a PPI pulse at a low voltage followed by a main shock at a much higher voltage, two shocking capacitors are usually required—one precharged to the low voltage and the other precharged to the high voltage. However, high voltage PPI pulses can be painful in and of themselves. Hence, low voltage PPI pulses are typically used instead, although the extra shocking capacitor is required along with correspondingly more complicated shocking circuitry. In this regard, note that capacitors used for generating conventional pacing pulses ordinarily cannot be employed to also generate low voltage PPI pulses, which typically require a somewhat higher voltage than the pacing pulses.
In addition, both techniques share a common disadvantage, at least as conventionally implemented. The inventors have found that, for a given PPI pulse voltage, the total pulse duration is typically set to a width far greater than actually necessary to achieve adequate pain inhibition, thus consuming more energy than otherwise required and unnecessarily depleting power reserves within the implanted device. Also, insufficient consideration has been given to selecting the electrodes for use in delivering the PPI pulses, even though the choice of electrodes can affect the degree of pain inhibition achieved with a given pulse energy and voltage as well as the amount of pain caused by the PPI pulse itself.
Accordingly, it would be desirable to provide techniques for determining preferred PPI pulse durations sufficient to achieve adequate pain inhibition so that pulse energy can be minimized and energy consumption reduced. It would also be desirable to provide various PPI pulse techniques that exploit the preferred pulse durations. It would also be desirable to identify preferred electrode combinations for use in delivering PPI pulses. It is to these ends that aspects of the invention are directed.